Many types of medicines are provided in fluid form, such as a solution or suspension of particles in a propellant or emulsion, and are adapted for oral inhalation by a patient. As one example, a canister might contain asthma medicine such as fluticasone propionate.
Inhaler devices are commonly used for delivery of a wide range of medicaments. Such an inhaler device normally comprises an actuator body that holds a canister of medicament, the canister being actuated by compression to deliver a dose of medicament through a mouthpiece to a user.
Canisters for holding a suspension or solution of a pharmaceutical substance in a propellant under pressure are well known. One such known canister to be used in an inhalation device comprises a can coupled with a metering valve. The metering valve consists of a body comprising a chamber, a valve stem (which extends from a head of the body) and a metering chamber, selectively communicable by the valve stem to the atmosphere via an L-shaped conduit within the valve body. The valve stem is axially displaceable between a first, extended position in which the metering chamber, and hence the canister, is closed to the atmosphere (since the L-shaped conduit is disposed wholly outside the metering chamber), and a second, depressed position, in which the metering chamber is in open communication with the outlet provided by the L-shaped conduit in the valve stem and through which a metered dose of propellant containing pharmaceutical substance is delivered. Inserted in the inhaler device, the valve stem is inserted into a stem receiving body, which is part of the actuator body. The patient presses down on the base of the canister, held in an inverted position in the actuator body, whereby the valve stem is forced against the stem receiving body in the inside of the actuator, and when the valve stem has been depressed sufficiently to open the metering channel, the inhaler will discharge a single dose. This is known as the firing of the inhaler device. This technology is well known and described in many parts of the prior art. For example in Respiratory Drug delivery, Ed Peter Byron, CRC press, 1991, and in Lung Biology in Health and Disease, Vol 94, Inhalation Aerosols, A J Hickey, Publisher Marcel Decker, 1996 and in Pharmaceutical Preformulation and Formulation, Ed Mark Gibson, Chapter 10, Inhalation Dosage Forms, IHS Health Group, 2001.
It is important for the patient to be able to ascertain the level of medicament remaining in the canister. This requirement is met by the use of a dose counter unit either attached to or part of the inhaler device. It is obvious that this counter must be accurate in that over-counting results in medicament being left in the inhaler unnecessarily resulting in more repeat prescribing with their resultant costs and inconvenience to patient. It also means that in an emergency situation the patient may not use the inhaler in the belief that it is empty when it is not. Conversely, undercounting could result in the inhaler being empty of medicament before the patient has a replacement, which could be dangerous for the patient and reduce patient compliance. Regulatory guidelines require the over and undercounting errors to be minimised.
An inhaler traditionally contains from 120 to 200 doses (shots) so any counter must be reliable, accurate and easy to read. There are many examples of dose counters in the prior art, some located near the valve region of the canister and some attached to the base end of the canister which due to the fact that the canister is in an inverted position in the inhaler mouthpiece means that the counter is on the top of the assembled inhaler and can be easily viewed.
In U.S. Pat. No. 4,817,822 a counter attached to the base end of the canister forming an inhaler canister assembly, is disclosed. A problem with the counter disclosed in U.S. Pat. No. 4,817,822 is that the counter actuation member is arranged in a non protected position, whereby it can be actuated by mistake or tampered with. Moreover, the inhaler canister assembly can be placed in the wrong orientation in the actuator body, whereby the inhaler device could be fired without actuating the counter.
In WO 02/067844 a counter attached to the base of the canister is disclosed where the counter actuator means extends along the whole periphery of the top edge of the actuator body. One problem with this design is that the counter actuation member could easily be tampered with. Moreover counters attached to canisters containing different medicament could easily be interchanged between different actuators and made to operate